Infectious diseases are a major, re-emerging health problem, accounting for 33% of the world mortality (17 million/year). The emergence of new virulent variants, widespread resistance to antibiotics, and increased worldwide travel are but a few factors that have increased the severity of the problem not only in developing countries, but also for people traveling to these regions, and for individuals with reduced immune functions. Mycobacterial infections continue to be a serious health threat, causing tuberculosis in otherwise healthy individuals, and causing disseminated infections in immuno-suppressed patients. The host defense mechanisms against these infections, including the effector functions of macrophages and parameters of protective immunity, need to be better understood. Such host defense mechanisms can manifest themselves as genetic determinants of innate resistance or susceptibility to infections in human populations, and in corresponding animal models. Using a genetic approach in mice, we have detected a major locus on chromosome 7, designated Trl3, which controls susceptibility to pulmonary tuberculosis in mice. In the first specific aim of this application, we propose to identify the gene and protein that control this differential susceptibility. This will be done by creating and characterizing a series of congenic and subcongenic mouse strains, in which Trl3 has been isolated by selective breeding. The minimal genetic interval will be carefully scrutinized for the presence of positional candidates that will then be systematically tested in transgenic mice in vivo and in macrophages in vitro. The function of the protein product of the gene and its role in macrophage and dendritic cell defenses against tuberculosis will be investigated. Independently, we have uncovered that a mutation in the transcription factor IRF8 causes extreme susceptibility to pulmonary tuberculosis in mice, and have identified a homozygote loss-of-function IRF8 mutation in a patient suffering from Mendelian Susceptibility to Mycobacterial Disease (MSMD). In the second aim of this application we propose to implement genome-wide methods, including transcript profiling with microarrays and chromatin immunoprecipitation (ChIP) and genomic DNA arrays (ChIP on chip) to systematically identify genes regulated by IRF8, and that may code for key defense mechanisms of macrophages against mycobacteria. A series of in vitro and in vivo experiments are also proposed to validate IRF8 regulation of the identified targets, and to evaluate their role in macrophage defenses against mycobacteria. We also propose to assess the role of these targets in human mycobacterial infections, by monitoring their integrity in a cohort of 300 patients suffering from MSMD.